1. Field of the Invention
The present invention relates to an optical pick-up apparatus comprising a light source emitting a light beam, an objective lens projecting the light beam onto an optical record medium as a fine spot, a movable member including a holding means for holding the objective lens movably in a focusing direction parallel to an optical axis of the object lens to perform a focusing control, a supporting means for supporting the movable member movably in a tracking direction perpendicular to said focusing direction as well as to a direction of an information track formed in the optical record medium, a driving means for driving the supporting means in the tracking direction to perform a tracking control as well as an access or seek control.
2. Related Art Statement
The optical pick-up apparatus of the kind mentioned in the preamble has been known and has been widely used in various optical information recording and/or reading apparatuses using optical record medium such as CD-ROM, magneto-optical disk and phase-change disk.
In the optical information recording and/or reproducing apparatus, there are provided three kinds of supporting mechanisms for moving a light spot with respect to an optical record medium, i.e., a mechanism for moving the light spot in the tracking direction across whole information tracks to perform an access, a mechanism for moving the light spot over a very small distance in the tracking direction to effect the tracking control, and a mechanism for moving the light spot in the focusing direction perpendicular to a plane of the optical record medium.
In order to reduce a size and a cost of the optical pick-up apparatus, there has be en proposed an optical pick-up apparatus in which the access control and tracking control are carried out by one and same supporting mechanism. For instance, Japanese Patent Laid-open Publication Kokai Sho 63-224037 teaches such an optical pick-up apparatus. FIGS. 11 and 12 are perspective view and side view, respectively showing the known optical pick-up apparatus disclosed in said Publication. An objective lens 50 is supported on a movable member 51, which is then supported by a pair of guide rods 52 slidably in a direction x which is perpendicular to directions y and z. It should be noted that the direction z is the focusing direction and the direction x is the access direction as well as the tracking direction. To this end, there are provided a coil 53 for performing the tracking control and access control and a magnetic circuit 54.
The objective lens 50 is held by a holder 59 and the holder is supported by a pair of leaf springs 56 movably in the focusing direction z, these leaf springs being secured to a fixing member 57 provided on a carriage 55. To the holder 59 is secured a focusing coil 58 which cooperates with the magnetic circuit 54.
In the above mentioned known optical pick-up apparatus in which both the tracking control and access control are carried out by the same driving means including the coil 53 and magnetic circuit 54, during the tracking control, the light spot has to be moved in the tracking direction x in a very precise manner in order that the light spot follows a very fine displacement of the information track. In the known apparatus shown in FIGS. 11 and 12, holes formed in the carriage 55 are constructed as slide bearings, and guide rails 52 are inserted in these slide bearings. Then, a coefficient of static friction .mu. between the slide bearings and the guide rails 52 has to be considered. When the coefficient of static friction .mu. is large, it is impossible to perform a precise and fine tracking control and the light spot could not follow an information track precisely and information could not be read and/or read accurately. In the known optical pick-up apparatuses, a value of the static friction between the slide bearings and the guide rails has not been mentioned at all. That is to say, there has not been proposed any indication in designing the slidable coupling between the slide bearings and the guide rails.
There has been proposed another type of optical pick-up apparatus in which an objective lens holder is supported movably in the tracking direction with respect to a movable member and the movable member is arranged movably in the tracking direction over a relatively large distance to perform the track access control. Such an optical pick-up apparatus may be called a dual-stage servo apparatus. In such a dual-stage servo optical pick-up apparatus, the above mentioned problem of the static friction does not occur. Therefore, a person skilled in the art could not derive any suggestion in designing the slidable coupling between the slide bearings and the guide rails from the dual-stage servo optical pick-up apparatus.